The present invention relates generally to the inspection of sheet material and, more particularly, to an improved apparatus for accurately and rapidly determining the contours of bent glass sheets.
Bent sheets of glass are commonly used as glazing closures for vehicles such as automobiles and the like. For such applications, the glass sheets must be bent to precisely defined curvatures or contours dictated by the configuration and size of the opening and the overall styling of the vehicle. In order to determine the accuracy of the shape imparted to the sheets, it has been customary to employ a checking fixture which is formed with a surface conforming to the contour desired in the glass sheets. A bent sheet to be inspected is placed upon the contoured surface of the fixture and, if in agreement or conformity therewith, is found to be acceptable. However, if the sheet is found to be "off-form", various arrangements have been employed to determine whether the deviation from the desired or ideal contour is within allowable tolerances. One simple process involved the insertion of feeler gauges at intervals between the marginal edge portions of the sheet and the surface of the checking fixture to ascertain whether the clearance therebetween was acceptable. It can be appreciated that this was a tedious and slow process, subject to errors in judgment and, because of modern-day production output on the order of about six bent pieces per minute, allowed the inspection of only a fraction of the finished pieces. Moreover, this procedure determined only whether the sheet being checked was acceptable or unacceptable, without any indication of the precise contour thereof.
Other known techniques that were somewhat more sophisticated employed a plurality of air actuated probes located about the marginal edge of the fixture and which projected through the surface of the fixture for engaging the underside of the sheet when placed on a form to indicate whether the deviation, if any, at spaced points along the marginal edge of the bent sheets fell within or beyond tolerable limits. The vertical positions of the several probes were determined by measuring back pressures from the sensors, which were connected to a system of lights and audible alarms that were actuated if any area of the sheet did not fall within acceptable tolerances. While such sensors were an improvement over the manual checking procedure, they were not completely satisfactory in that the data from such sensors depended upon the air back pressure and the resulting signals were non-linear, causing any deviation to be amplified. Also, as with the feeler gauges mentioned above, such devices determined only whether or not the bent sheet fell within acceptable or unacceptable limits and could not give an accurate representation of the actual contour of the glass periphery.
Still another known checking practice involves the use of linear potentiometers as sensing probes to more accurately ascertain the actual contour of the sheet being inspected. However, this known technique requires calibration of each probe sensor, one after another, to arrive at an ideal or desired contour when preparing the fixture for glass inspection. Not only must this be done each time a production run of differently contoured sheets is made, but also periodically during production in order to ensure accuracy with consequent production down-time and increased production costs.